Thermal signature control structures

ABSTRACT

Subwavelength conducting particles can be arranged on conducting surfaces to provide arbitrary thermal emissivity spectra. For example, a thermal emissivity spectrum can be tailored to suppress a thermal signature of an object without sacrificing radiative cooling efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/507,496, filed May 17, 2017, entitled THERMALSIGNATURE CONTROL STRUCTURES, which is herein incorporated by referencein its entirety.

BACKGROUND AND SUMMARY

Subwavelength conducting particles can be arranged on a conductingsurface to provide a structure with a selective absorption spectrum forelectromagnetic radiation. See, e.g., A. Moreau et al,“Controlled-reflectance surfaces with film-coupled colloidalnanoantennas,” Nature 492, 86-89 (December 2012); and D. R. Smith etal., U.S. Pat. No. 9,606,414; each of which is herein incorporated byreference. An example of such a structure, sometimes referred to as ametasurface, is shown in FIG. 1A-1B. In the example, small conductingparticles 101 are arranged on a conducting surface 102, with adielectric spacer layer 103 interposed between each particle and theunderlying surface (FIG. 1A shows an above view of the surface, whileFIG. 1B shows a close-up cross section of a single particle). Eachconducting particle has a flat lower surface and forms a planar gapregion between the conducting particle and the conducting surface (inthis example, the particles are cubes, but any shape having a flat lowersurface would suffice). The planar gap region supports a Fabry-Perotresonance mode, with an exemplary electric field pattern for the modeseen in the close-up of FIG. 1B.

As explained in the references cited above and in further detail below,the resonant wavelength of the Fabry-Perot mode, and thus the absorptionspectrum for the structure, can be designed by selecting an appropriategeometry for the structure, e.g. by appropriate selection of the lengthand thickness of the planar gap region. For example, a resonantwavelength of about 11 microns may be obtained for gold cubes with anedge length of 500 nm, arranged on an underlying gold surface with a 20nm silica spacer layer. In typical embodiments, the length of the planargap region might range from about one-half to one-twentieth of theresonant wavelength, depending on the thickness of the gap region andthe materials used.

The present application relates to metasurfaces that are structured toprovide a selected thermal emissivity spectrum. By Kirchoff s radiationlaw, the spectrum of thermal emissivity of an object is identical to theabsorption spectrum for the object. Thus, by designing the sizes ofconducting particles and their arrangements on one or more conductingsurfaces, an arbitrary thermal emissivity spectrum can be obtained. Forexample, the structure can be designed to significantly suppress anobserved thermal signature of an object (e.g. as viewed with a thermalinfrared detector) without significantly sacrificing radiative coolingefficiency. These metasurfaces can be fabricated using colloidal orphotolithographic processes, and can be applied onto the surface of anyobject (e.g. by painting or lamination) to tailor the thermal emissivityspectrum of that object.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1B depict an example of a metasurface.

FIGS. 2A-2C depict thermal emissivity spectra.

FIGS. 3A-3B depict an example of tailoring thermal emissivity.

FIG. 4 depicts an example of a constituent unit for a composition.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here.

FIGS. 2A-2C illustrate an example of designing a thermal emissivityspectrum for a metasurface. In this example, the thermal emissivityspectrum is designed to significantly suppress a thermal signature of anobject (e.g. by a factor of 10 or more), while maintaining significantradiative cooling efficiency. Consider, for example, an object having atemperature of 50° C. If the object were a blackbody (i.e. a perfectabsorber), it would have a blackbody thermal emissivity 201 as afunction of wavelength, as shown in FIG. 2A.

Suppose that the object is remotely observed with a thermal infrareddetector. As shown in FIG. 2B, the atmospheric transmission spectrum 210of thermal infrared radiation is highly wavelength-dependent, withsignificant transmission in particular transmission bands (e.g. 3-to-5microns, 8-to-13-microns) and strong atmospheric absorption at otherwavelengths. Thus, the blackbody emissivity spectrum 201 would translateinto an detected radiance 221 at the thermal infrared detector, as shownin FIG. 2C. The spectral integral of this detected radiance is oneuseful measure of the thermal signature of the observed object.

Now, suppose that the object is covered with a metasurface that isdesigned to have a different thermal emissivity spectrum 202 as shown inFIG. 2A. Here, the emissivity spectrum is designed to be substantiallyreduced relative to the blackbody spectrum 201 in a selected spectralrange, in this case, a spectral range corresponding to the8-to-13-micron atmospheric transmission band (the multiple peaks areindicative of multiple resonant wavelengths for the conducting particlesarranged on the metasurface, as further discussed below). This modifiedthermal radiance 202 translates into a modified detected radiance 222 atthe thermal infrared detector, as shown in FIG. 2C. Again using thespectral integral of the detected radiance as a useful measure of thethermal signature of the observed object, it can be seen that themetasurface-covered object has a significantly reduced thermalsignature; in this case, the thermal signature is reduced by a factor ofabout 12.

At the same time, FIG. 2A illustrates that the radiative cooling rate,or radiative cooling efficiency, of the object is not comparablysuppressed by the metasurface cover. The radiative cooling efficiencycan be expressed as the spectral integral of the thermal emissivityspectrum over all frequencies. Outside of the selected spectral rangefor emission suppression, the emissivity spectrum is not substantiallyreduced relative to the blackbody spectrum; in this case, the overallradiative cooling efficiency is therefore only reduced by about 50%.

With a thermal emission spectrum designed as in FIGS. 2A-2C, significantradiative cooling of a hot object can be achieved while substantiallyconcealing the heat of the object from a distant thermal infrareddetector, by making the object appear to be substantially cooler thanits actual temperature. Consider, for example, a 10 cm spherical objectplaced in a 20° C. ambient environment, and suppose that the object isexposed to a 1000 W heat source. If the object were a blackbody (perfectabsorber), it would reach a steady state temperature of about 240° C.,and this would be the apparent temperature of the object as seen by athermal imager. If the object were a whitebody (perfect reflector), itwould have an apparent temperature of only 20° C. (the ambienttemperature), but the actual steady state temperature would besignificantly higher at 414° C. Coating the object with a metasurfacedesigned to have a thermal emission spectrum as in FIGS. 2A-2C yields acompromise of the two extremes, in that the object would not become ashot as the whitebody (actual steady state temperature of about 297° C.,versus 414° C. for the whitebody), and it would not appear as hot as theblackbody (apparent temperature of 53° C., versus 240° C. for theblackbody).

While the above discussion of FIGS. 2A-2C describes the thermalsignature suppression in terms of an emissivity spectrum multiplied byatmospheric transmission as a function of wavelength (and thenintegrated over all wavelengths), in another approach, the thermalsignature is instead defined in terms of an emissivity spectrummultiplied by a thermal detector response as a function of wavelength(and then integrated over all wavelengths). In yet another approach, thethermal signature is instead defined in terms of an emissivity spectrumthat is integrated over a selected spectral range, which could be arange of atmospheric transmission or a range of detector response for athermal infrared detector. In many contexts, these different approacheswill yield comparable results as they commonly reflect inherentlimitations in the remote detection of thermal signatures.

The thermal emissivity spectrum of the metasurface can be designed byarranging a plurality of differently-sized conducting particles on aconducting surface. Then the particles would have a set of resonantwavelengths corresponding to a set of sizes of the conducting particles.The thermal emissivity may be thereby enhanced within linewidths of theresonant wavelengths, and reduced outside of these linewidths. Theheights of the resonant peaks can be increased or decreased by changingthe relative concentrations (i.e. surface densities) of particles ateach of the different sizes, while the widths of the resonant peaks bycan be increased or decreased by, for example, using trapezoidal (asopposed to cubic or cuboidal) particles. Thus, by appropriate selectionof the sizes, shapes, and concentrations of the various conductingparticles on the conducting surface, the thermal emissivity spectrum canbe practically custom-tailored.

An example of this custom tailoring is shown in FIGS. 3A-3B. FIG. 3Adepicts an electron microscope image of a metasurface that is populatedwith an array of particles having four different sizes 301 (violet), 302(blue), 303 (green), and 304 (red). The thermal emission spectrum forthis metasurface, shown in FIG. 3B, features four prominent emissionpeaks 311-314 corresponding to the four different sizes 301-304. Thefour sizes were selected so that two emission peaks would be situatedbelow a selected spectral band 320 and two emission peaks would besituation above the selected spectral band 320; consequently, thermalemission is suppressed within the selected spectral band 320, due to anabsence of resonances therein.

While the above discussion has focused on laminar embodiments having asingle conducting surface with an arrangement of conducting particlesdistributed thereon, composition embodiments provide multiple conductingsurfaces, each the surface of a larger conducting particle, with smallerconducting particles distributed thereon. An example is shown in FIG. 4,which depicts a constituent unit 400 for a composition, the constituentunit including a larger conducting particle 401 having a plurality ofsmaller conducting particles 402 arranged on its surface. Because thelarger conducting particle is significantly larger than each of thesmaller conducting particles, the surface of the larger conductingparticle is locally planar under each of the smaller conductingparticles, providing planar regions between the larger particle and eachsmaller particle just as with the laminar embodiments. In typicalscenarios, the larger particles might be as small as 10 microns or aslarge as a 2-3 millimeters, while the smaller particles might be assmall as 200 nanometers or as large as 5 microns.

In some embodiments, the structure is covered by a layer that protectsand/or conceals the underlying structure. For example, the structure maybe covered with ZnO or FeO microparticles or nanoparticles that scattervisible light (e.g. to create a paint-like appearance) but aresubstantially transparent to infrared light, allowing the underlyingstructure to function. In some approaches, the ZnO or FeO particles maybe embedded in an infrared-transparent binder or matrix material.

Laminar embodiments can be mounted on a thin flexible structure (e.g. apolymer film such as Kapton), and then this laminar structure can beapplied as a cover to an object of interest by “wallpapering” the object(e.g. using an adhesive to attach the laminar structure to the surfaceof the object). On the other hand, composition embodiments can be mixedwith suitable binders, and then this composition can be applied as acover to an object of interest by “painting” the object (e.g. brushing,rolling, or spraying the composition onto the surface of the object).

Laminar embodiments may be fabricated by either by colloidal assembly orby photolithography. In a colloidal assembly approach, the conductingparticles randomly self-assemble on the conducting surface (e.g. as inFIG. 1A); to provide a desired surface density of variously-sizedparticles, the colloidal suspension of the conducting particles caninclude selected concentrations of the different particles. In aphotolithographic approach, a lift-off process can be employed thatentails depositing a negative photoresist on the dielectric spacerlayer; exposing the photoresist with the desired pattern of conductingparticles; evaporating gold onto the exposed photoresist; and thendissolving the photoresist to leave behind the conducting particles.After the conducting particles are arranged on the conducting surface(either by colloidal assembly or by photolithography), the arrangementmay be covered with a layer of infrared-transparent material to protectand/or conceal the structure. To provide a thin flexible structure for“wallpapering,” a thin flexible layer may be placed on a substrate, withthe conducting surface deposited on top of the thin flexible layer;then, after the conducting particles have been arranged on theconducting surface, the thin flexible layer can be peeled off of thesubstrate.

Composition embodiments may be fabricated by colloidal assembly, e.g. byplacing the larger particles in a colloidal suspension of the smallerparticles and allowing the smaller particles to self-assemble on thesurfaces of the larger particles. Again, to provide a desired surfacedensity of variously-sized particles, the colloidal suspension of theconducting particles can include selected concentrations of thedifferent particles; alternatively, the composition can be made up ofseveral batches of larger particles, the first batch assembling smallerparticles of a first size on the larger particles, the second batchassembling smaller particles of a second size on the larger particles,and so on. The assembled particles can then be mixed with a suitableinfrared-transparent binder for “painting.”

All of the U.S. patents, U.S. patent application publications, U.S.patent applications, foreign patents, foreign patent applications andnon-patent publications referred to in this specification and/or listedin any Application Data Sheet, are incorporated herein by reference, tothe extent not inconsistent herewith.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is: 1-33. (canceled)
 34. A composition, comprising: aplurality of conducting larger particles; and for each of the conductinglarger particles, a plurality of conducting smaller particles arrangedon a surface of the larger particle, each conducting smaller particlehaving a flat surface and forming a planar gap region between the largerparticle and the smaller particle; wherein the pluralities of conductingsmaller particles are arranged to provide a selected thermal emissivityspectrum for the composition.
 35. The composition of claim 34, whereinthe larger particles have dimensions greater than about 10 microns andsmaller than about 2-3 millimeters.
 36. The composition of claim 34,wherein the smaller particles have dimensions greater than about 200nanometers and smaller than about 5 microns.
 37. The composition ofclaim 34, further comprising: an infrared-transparent binder.
 38. Thecomposition of claim 37, further comprising: a diluent or solvent. 39.The composition of claim 34, wherein the selected thermal emissivityspectrum is a thermal emissivity spectrum that reduces a thermalsignature of the composition by a first factor while reducing aradiative cooling efficiency of the composition by a second factorsubstantially smaller than the first factor.
 40. The composition ofclaim 39, wherein the reduced thermal signature is less than one-third,one-fifth, or one-tenth of a thermal signature of a blackbody having asame actual temperature as the composition.
 41. The composition of claim40, wherein the radiative cooling efficiency is greater thanone-quarter, one-half, or three-quarters of a radiative coolingefficiency of the blackbody having the same actual temperature as thecomposition.
 42. The composition of claim 39, wherein the thermalsignature corresponds to thermal radiance integrated over a selectedspectral range of infrared wavelengths.
 43. The composition of claim 39,wherein the thermal signature corresponds to a product of thermalradiance times atmospheric transmission as a function of wavelength,integrated over all infrared wavelengths.
 44. The composition of claim39, wherein the thermal signature corresponds to a product of thermalradiance times thermal detector response as a function of wavelength,integrated over all infrared wavelengths.
 45. The composition of claim39, wherein the radiative cooling efficiency corresponds to thermalradiance integrated over all infrared wavelengths.
 46. The compositionof claim 34, wherein the selected thermal emissivity spectrum provides:an apparent temperature of the composition that is substantially lessthan an actual temperature of the composition; and an actual radiativecooling rate that is substantially greater than an apparent radiativecooling rate.
 47. The composition of claim 46, wherein the apparenttemperature corresponds to a temperature of a blackbody having ablackbody thermal radiance in a selected spectral range equivalent to anactual thermal radiance of the composition in the selected spectralrange.
 48. The composition of claim 46, wherein the apparent radiativecooling rate corresponds to a radiative cooling rate of a blackbodyhaving the apparent temperature.
 49. The composition of claim 34,further comprising, for each of the larger particles: a spacer layeroccupying the planar gap region between the larger particle and eachsmaller particle.
 50. The composition of claim 49, wherein the spacerlayer is a layer of material transparent to thermal infrared radiation.51. The composition of claim 50, wherein the material is ZnS or ZnSe.52. The composition of claim 34, wherein the smaller particles are noblemetal particles.
 53. The composition of claim 52, wherein the noblemetal particles are gold particles.
 54. The composition of claim 34,wherein each of the smaller particles has a resonant wavelength selectedfrom a set of resonant wavelengths, the set of resonant wavelengthscorresponding to a set of sizes of the smaller particles.
 55. Thecomposition of claim 54, wherein the set of sizes of the smallerparticles is a set of lengths of the planar gap regions.
 56. Thecomposition of claim 54, wherein the selected thermal emissivityspectrum includes: one or more spectral ranges of enhanced thermalemissivity that include the set of resonant wavelengths; one or morespectral ranges of suppressed thermal emissivity that exclude the set ofresonant wavelengths.
 57. The composition of claim 56, wherein the oneor more spectral ranges of suppressed thermal emissivity include aselected spectral range, and the set of resonant wavelengths includesone or more resonant wavelengths below a lower wavelength limit of theselected spectral range or above an upper wavelength limit of theselected spectral range.
 58. The composition of claim 57, wherein theset of resonant wavelengths one or more lower wavelengths below thelower wavelength of the selected spectral range and one or more upperwavelengths above the upper wavelength of the selected spectral range.59. The composition of claim 34, wherein, for each of the largerparticles, the plurality of smaller particles is a colloidal assembly ofthe smaller particles on the larger particle.
 60. The composition ofclaim 42, wherein the selected spectral range is a range of atmospherictransmission of thermal infrared radiation. 61-76. (canceled)
 77. Thecomposition of claim 34, wherein the pluralities of conducting smallerparticles are arranged according to a specific arrangement selected toprovide the selected thermal emissivity for the composition with respectto a specific radiative cooling efficiency for the composition.
 78. Thecomposition of claim 77, wherein either or both a size and a shape ofeach of the pluralities of conducting smaller particles are selected toprovide the predetermined thermal emissivity spectrum for thecomposition.
 79. The composition of claim 77, wherein the pluralities ofconducting smaller particles are arranged by colloidal assembly.
 80. Thecomposition of claim 77, wherein the pluralities of conducting smallerparticles are arranged by photolithography.